The present invention relates generally to implantable cardiac stimulation devices and, more particularly, to a simplified implantable lead for a combination implantable cardioverter/defibrillator (ICD) with bradycardia support pacing system adapted to transmit electrical signals using an improved distal end portion of the lead, the distal end performing shocking, pacing and sensing functions using a single electrode.
A depolarization signal (a small electrical impulse) is generated by most muscle tissue as such tissue contracts. Thus, the beating or contracting of a human heart is manifest by appropriate depolarization signals evidencing: the contraction of the atria, referred to as the P-wave, and the contraction of the ventricles, referred to as the R-wave (or the QRS complex). The sequence of P-waves followed by R-waves thus comprises an electrogram or electrocardiogram signal that can be monitored by appropriate electrical circuits to indicate the status of the heart.
An implantable pacemaker includes sensing circuits that monitor the heart by looking for the occurrence of P-waves and/or R-waves, and pacing circuits that stimulate the heart with an appropriate electrical stimulation pulse in the event that a depolarization signal is not sensed within a prescribed time period. In this way, if the heart does not beat naturally within the prescribed time period, then an electrical stimulation pulse is provided to force the heart muscle tissue to contract, thereby assuring that the prescribed minimum heart rate is maintained.
An implantable cardioverter-defibrillator (ICD) typically includes sensing and pacing circuits to provide electrical stimulation pulses aimed at responding to slow intrinsic (natural) cardiac rates or asystole (a non-beating heart). The pacing circuits may also provide appropriate electrical stimulation pulses, typically in a prescribed burst or pattern, aimed at terminating rapid intrinsic rates (tachyarrhythmias or tachycardias).
With any bioelectric stimulation device, it is essential to determine accurately the ability of that device to accomplish the task for which it was designed. If nature were truly constant, and in-finitely small steps were used, theoretically it would be possible to define a limit, or threshold, below which no activation would occur and above which activation would occur 100% of the time (see FIG. 1A). In biological systems, such constancy is not possible. Instead, a balance point is the norm, at which activation occurs 50% of the time (E50 shown in FIG. 1B).
In addition, even with all external variables constant, there remains the inherent problem of biovariability, both subject-to-subject and time-dependent, thus yielding a sigmoidal-shaped curve (FIG. 1B) for finding the probability of success of a stimulus. For the investigator working with implantable defibrillators, variability is abundant, with some variables determinable, but most not, as they appear to vary either xe2x80x9crandomlyxe2x80x9d or xe2x80x9cchaotically.xe2x80x9d Despite major design improvements and mathematical expertise in the last two decades, our ability to understand the mechanisms of this variability has changed little since the classical experiments performed in the 1930s. Nevertheless, there have been several attempts to accurately assess defibrillation efficacy within a reasonable window of probability (also called the probability of success curve for defibrillation).
It is important to recognize that there is variation in the probability of any system to defibrillate at a particular instant in time (FIG. 1B). The spectrum of factors which may contribute to this variability are poorly understood. For example, the same setting on a defibrillator will fail on one attempt but be successful a few seconds later, with no obvious change in any measured variable.
There has been some suggestion that the mechanisms which alter efficacy due to the stochastic processes might be xe2x80x9crandomxe2x80x9d or xe2x80x9cchaoticxe2x80x9d. Although this has not been resolved, it seems implicitly clear that, as myocytes are not instantaneously depolarized and spontaneously repolarized, once a particular wave pattern of activation is established (at a given interval in time), in the next several milliseconds the pattern cannot be truly random since, to be truly random, all myocytes must have an equal chance of being reactivated. Clearly, those which are totally depolarized or are in the early phases of the activation will be in a refractory state and will not be reactivatable. Thus, for the subsequent several milliseconds, the movement of the wavefront(s) cannot be random. Therefore, although the tenant of randomness remains possible for the pattern in the first instant selected, a second selection within a short period of time thereafter cannot be random. Also, there may be some factors that influence defibrillation which are determinable.
The previous theory applies to atrial and ventricular defibrillation.
An ICD must perform at a minimum sensing, pacing, and defibrillation. Common ICDs use a multitude of electrodes to sense/pace and defibrillate. However the conventional approach, with dedicated electrodes for defibrillation, pacing, and sensing result in a complex, large lead body (greater than xcx9c10 F). The large size of the lead results mainly because of insulation between the different conductors. In addition, due to the presence of a pacing dipole at the tip of a conventional lead, the defibrillation coil may be 2 cm away from the apical area when used in the right ventricle. This results in a low potential gradient near the apex and causes higher defibrillation thresholds.
Typical of the prior art in this regard, the following U.S. patents all disclose endocardial leads used for both sensing and pacing:
Each of the following U.S. patents discloses an electrode assembly for an implantable lead:
and the patents to Soukop et al., Yamasaki, and Bolz et al. specifically disclose porous electrode constructions for increasing the effective surface area of the electrode. It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.
The present invention relates to an implantable stimulation system and lead which is adapted to transmit electrical signals between a return electrode and an xe2x80x9cintegratedxe2x80x9d distal electrode capable of providing pacing pulses and shocking pulses, as needed, to stimulate selected body tissue, in addition to receiving cardiac signals. Whereas the prior art required separate electrodes for pacing/sensing, shocking, the present invention advantageously integrates all these functions into a single electrode, significantly simplifying the lead""s construction.
Although the previous background material is directed towards ventricular defibrillation, the same principles of the invention can be applied to atrial defibrillation.
The present invention is compatible with an implantable cardioverter defibrillation (ICD) device, which includes circuitry for sensing intrinsic depolarization signals of the patient""s heart, first and second stimulating circuitry for generating electrical pacing pulses and defibrillation pulses, respectively, and transmitting such pulses to the patient""s heart, and an electrically conductive enclosure (i.e., the housing or xe2x80x9ccasexe2x80x9d electrode) protectively supporting and encompassing the sensing circuitry, and the first and second stimulating circuitry.
The present invention is directed towards an improved implantable lead having a single biocompatible electrode at the distal end of the lead which engages with the body tissue in the right ventricle of the patient""s heart. The implantable lead connects the electrode to the ICD for transmitting the sensing, pacing, and defibrillation pulses, respectively.
The distal electrode includes an end cap in electrical continuity with the lead, the end cap having a diameter in the range of about 1-3 mm and a length in the range of about 2-10 mm. Being of the same diameter as the electrical lead, the end cap preferably has a porous outer surface with an irregular relatively large surface area and an outer diameter substantially the same as that of the electrical lead. The distal electrode further includes a coil in electrical continuity with the end cap, the coil having a length suitable for placement in the ventricle, typically about 2-10 cm.
In short, a simple defibrillation/sense/pace electrode system is being proposed by the invention. A single coil electrode with an end cap is to be placed in the right ventricle and a platinized (or titanium nitride coated) end cap incorporated into the system to help increase the surface area of the tip. The case electrode for the ICD is the only other electrode in this first embodiment. Pacing and sensing is done between the coil and the device""s enclosure similar to a unipolar pacemaker. Defibrillation is done between the coil and the ICD""s enclosure.
In a second embodiment, a second coil electrode may be placed in the superior vena cava (SVC), which may act as the return electrode in lieu of the case electrode described above. In this embodiment, the lead requires only two conductors.
A primary feature, then, of the present invention is the provision of a simplified lead for an implantable cardioverter/defibrillator (ICD) adapted to transmit electrical signals between a proximal end portion of the lead and a distal end portion of the lead and to thereby stimulate selected body tissue.
Another feature of the present invention is the provision of such a simplified lead according to which a single coil electrode with an end cap is placed in the right ventricle, the enclosure for the ICD being the only other electrode for the system.
Still another feature of the present invention is the provision of such a simplified lead which utilizes a platinized or titanium nitride coated or otherwise coated end cap for increasing the effective surface area of the tip portion.